Two piece aluminum containers are used extensively for packaging beverages such as beer, carbonated soft drinks and other beverages such as tea. The two piece containers (cans) are comprised of a can body, which is typically made from lightweight materials, such as aluminum or aluminum alloys, and a can lid, which forms the top of the container. After the beverage has been introduced into the internal cavity formed by the can body, the can lid is placed on the open end at the top of the can body, and the can body and can lid are joined together to form a sealed container for the beverage contained therein.
The can body is manufactured by a method called drawing and ironing. The process begins with a plurality of generally circular pieces being punched from a flat sheet of material, which is typically packaged in large rolls. Each blank is then drawn to produce relatively shallow cup-shaped pieces. Next, in a sequence of ironing operations, the cup is placed over a punch and forced through a set of dies to stretch and thin the side walls until the cup is of approximately the desired can height. After the side-walls have been drawn, the bottom portion of the can is still flat, unworked and of about the same thickness as the original sheet metal.
The bottom profile of a can body is typically formed as the last step, in a pressing process that draws material to the required shape and dimensions. The most common bottom profile for a can is a dome bottom, wherein a large portion of the can bottom is formed into a spherical inwardly concave dome, with a convex annular portion, or foot formed around the outer diameter of the can bottom on which the can stands when it is upright on a horizontal surface. This configuration has been found to resist deformation of the can bottom under internal pressure, provides sufficient strength to hold the formed can and its contents in an upright position, and resist ruptures and bulging. The can bottom dome is formed when a punch, sometimes referred to as punch nose tooling, which is positioned in the interior of the can body is forced against an end-forming die, sometimes called a dome plug, located on the outside of the can body, to form the generally upwardly extending dome configuration that becomes the bottom of the can. After the can body has been formed, the open top of the can is trimmed to ensure a smooth continuous flat top edge to ensure a continuous seal with the can lid.
The need for a strong can bottom has required substantial thickness be retained in the bottom to achieve desired performance. If the can bottom is not sufficiently strong, the central dome area may reverse shape, becoming convex if the filled can is subject to high pressure. The resistance of a can bottom to reversing is one criteria which is used to measure the strength of a particular can bottom profile. This pressure is referred to as the “dome reversal pressure” or DRP. Design changes that increase the dome reversal pressure make the can more robust in higher pressure situations, such as in pasteurizing equipment.
Another criteria for measuring the strength of a particular bottom profile is drop resistance, which is the capability of a container bottom to resist a downward bulge when dropped from a height.
The pressure at which the can dome reverses or can bottom otherwise bulges or fails in response to dropping may be dependent upon can bottom design, gauge thickness, and the internal pressure of the can, which in turn is directly related to a variety of factors, such as the formula of the beverage in the can, carbonation of the beverage in the can, and ambient temperature conditions.
In some circumstances, the standard cans previously used in the industry, such as those disclosed in U.S. Pat. No. 6,182,852, may fail, especially in areas with temperature or pressure extremes, or when beverages that exert greater internal pressure are placed in the cans. Thus, there remains a need for improved container bottom profiles that show an increased resistance to failures. Further, there exists a need for improved tests so that failures in the consumer environment can be more accurately predicted, anticipated, and therefore prevented by designing cans that meet market needs better.